What does "off-the-shelf" mean?
The dictionary's definition of off-the-shelf is

used to describe a product that is available immediately and does not need to be specially made to suit a particular purpose

The same dictionary provides several examples

You can purchase off-the-shelf software or have it customized to suit your needs.
If you have complex needs, we don't recommend that you buy software off the shelf.
For this, off-the-shelf algorithms included in the robot's programming libraries are used.

So, your intuition is correct! An off-the-shelf model, software, product, etc., is any model, software or, respectively, product that would be easily (or immediately) available or applicable to the specific context, but, at the same time, it may also be applicable to many other contexts or problems.
An off-the-shelf convolutional neural network is thus a typical or standard CNN that can be applied immediately in that context (but that is potentially applicable to many other contexts or problems). Examples of CNNs that could be used as off-the-shelf models are AlexNet or LeNet-5, but the actual choice depends on the context and needs.
An off-the-shelf model can also be a baseline model (i.e. a very simple model that is used in experiments as the model that every other model should outperform), but not necessarily.
Alex Graves uses this term/expression in his paper Practical Variational Inference for Neural Networks.